A reflecting plate is used for reflecting a visible light source. The reflecting plate contains a diffuse reflecting plate applied with white paint or containing diffusing beads, a polished metal plate, and a reflecting plate in the form of a thin film obtained by stacking metal atoms on a substrate.
These visible light reflecting plates are widely used in various applications such as a backlight unit for a liquid crystal display, a reflecting plate for an indoor fluorescent lamp, a reflective layer of a recording medium such as a CD or a DVD, and a vehicle or indoor mirror.
Recently, large-size flat panel displays have increased in demand in place of current CRT displays. Herein, the large-size flat panel display normally represents a normal type (4:3) or wide type (16:9) flat panel display having a display diagonal of 28 inches or more.
The large-size flat panel displays mainly include a plasma display and a liquid crystal display but, in terms of low power consumption and light weight, the large-size flat panel liquid crystal displays have rapidly increased in demand.
Since the large-size flat panel liquid crystal display is not the self light emitting type like the plasma display, a light emitting portion called a backlight unit is inevitably required on a back side. The backlight unit is provided therein with a cold cathode fluorescent lamp (CCFL) for light emission, a reflecting plate, and various optical sheets.
Normally, a flat panel liquid crystal display of 25 inches or less employs, as a structure of a backlight unit, the sidelight type where a light guide plate made of transparent resin is used with a CCFL disposed on the side. However, in the large-size flat panel liquid crystal display of 28 inches or more, a light guide plate to be used increases in size and therefore increases in thickness and weight and thus in cost. Accordingly, the large-size flat panel liquid crystal display employs the right-under type where a CCFL is disposed right under a liquid crystal panel with a reflecting plate arranged behind it.
Presently, as the reflecting plate for the large-size flat panel liquid crystal display backlight unit, use is made of a diffuse reflecting plate applied with white paint or containing diffusing beads.
The reason for using such a reflecting plate is that because of diffuse reflection, there is no directivity in reflecting light and therefore it is possible to minimize luminance nonuniformity in the display screen. However, since no directivity exists in reflecting light, much light disappears on the wall surfaces and so on and thus the utilization efficiency of light is low. Consequently, for example, in a flat panel display having a screen diagonal of 30 inches, it is necessary to use at least 12 CCFLs for achieving the luminance. As a result, there is a drawback of increasing the power consumption.
In order to reduce the number of CCFLs to be used as much as possible to thereby diminish the power consumption, a reflecting plate is required that can control the light reflection direction, i.e. light directivity, to utilize the light efficiently.
In order to give the directivity to reflecting light, it is necessary to use reflection by a metal surface. With respect to a metal reflecting plate, it has been demonstrated in natural science that an angle formed by a light incidence direction and a perpendicular to a reflective surface, i.e. a light incidence angle, and an angle formed by a light outgoing direction and the perpendicular to the reflective surface, i.e. an outgoing angle, are equal to each other and, therefore, the reflection direction can be freely controlled based on a design for the reflective surface.
Normally, use is made of aluminum or silver as a metal component for uniformly reflecting visible light with respect to respective wavelengths. Although copper or gold is a metal, copper or gold has a property of absorption of low-wavelength light by itself. As a result, reflecting light is colored so that it is not preferable.
A metal reflecting plate is a reflecting plate using a metal and is increased in reflectance by rolling, polishing, and aging a case metal ingot. In this case, since silver is quite expensive, an aluminum ingot is normally used.
With respect to an aluminum reflecting plate formed by rolling an aluminum ingot, Japanese Unexamined Patent Application Publication (JP-A) No. 2001-281426 (hereinafter referred to as reference document 1) reports about ultraviolet reflection that the reflectance of 350 nm ultraviolet light becomes 95% by setting an aluminum purity to 99 mass % or more, setting the ratio of (220) plane/(200) plane or the ratio of (111) plane/(200) plane to 1.0 or more in X-ray diffraction intensity ratio, and setting the ratio of the total area of (110) planes and (111) planes in the surface of the plate to 30% or more, but does not report a reflectance exceeding 92% with respect to light in the range of 400 nm or more being the visible light range.
In order for a reflecting plate for a large-size flat panel liquid crystal display backlight unit to use, efficiently and without luminance nonuniformity, light emitted from a CCFL serving as a light source, the reflecting plate itself needs to be formed with a fine surface structure that is calculated to control the direction of reflecting light. When use is made of an aluminum reflecting plate formed by rolling an ingot, it is necessary, for forming the fine surface structure, to carve such a structure per plate and further it is difficult to accurately control the surface roughness of a reflecting substantially flat or curved surface.
Further, since aluminum has a high specific gravity of 2.71, the weight of the reflecting plate itself becomes large in the case of the aluminum reflecting plate formed by rolling the ingot and therefore it is difficult to actually use it as the reflecting plate in the backlight unit for the large-size flat panel liquid crystal display requiring an reduction in weight.
For reducing the weight of a reflecting plate itself, a reflecting plate has been developed which uses a metal thin film. As a base material (substrate) of the reflecting plate, it is preferable to use a plastic material in view of the point that it is lightweight, enables formation of a fine surface shape with high accuracy, and further enables mass production at a low price by the use of a molding method such as injection molding.
There is a reflecting plate using a thin film of silver as a metal excellent in reflectance. This silver thin film reflecting plate has recently started to be used as a reflecting plate excellent in reflection efficiency, i.e. having a reflectance of 97%.
However, silver itself is expensive and has a drawback of reacting with a very small amount of sulfide (sulfur dioxide or hydrogen sulfide) in the air so as to be blackened and reduced in reflectance. Further, it is necessary to apply various coatings on the silver thin film so that the silver thin film reflecting plate becomes more expensive and thus is not suitable as the reflecting plate for the backlight unit for the large-size flat panel liquid crystal display having a large area.
Therefore, an aluminum thin film is suitable for the reflecting plate for the large-size flat panel liquid crystal display backlight unit.
For forming the aluminum thin film, use is normally made of a deposition method in which aluminum is sublimated in high vacuum and adhered to a base material substrate. The aluminum thin film formed by this method follows a fine surface shape formed on the base material substrate and therefore it is possible to reproduce the fine surface shape as it is to thereby control the directivity of reflecting light.
However, the aluminum thin film formed by the conventional deposition method is amorphous so that configuration on an atomic level is not elaborate, and therefore, it is not possible to improve the reflectance to 92% or more.
For improving the reflectance of the aluminum thin film formed by the deposition method, a method is to realize a reflectance of 95% or more by stacking in layers a dielectric such as SiO2 or MgF on the aluminum surface (reflection increasing coating).
However, a problem has been that the method is not practical because those films should be formed in a number of layers on the aluminum thin film so that the members and processes increase in number to raise the cost and because it is quite difficult to uniformly perform such coating.
Further, it has conventionally been difficult to form a practical flat or curved surface portion having a surface roughness (Ra) of 40 nm or less and, in addition, difficult to implement uniform formation of a deposition thin film itself over the whole surface of the substrate following the increase in size thereof. Consequently, even if the calculated fine surface shape is formed to control the reflecting light, the luminance nonuniformity cannot be completely controlled.
As measures for forming an aluminum thin film, a sputtering method has conventionally been known in which argon ions produced from an argon plasma are irradiated onto an aluminum target under reduced pressure to thereby irradiate aluminum ions onto a base material substrate placed at a counter electrode so as to stack the aluminum ions thereon.
With respect to the aluminum thin film formed by such a conventional sputtering method, since the aluminum ions having energy are irradiated and stacked, the aluminum formed into the film is crystallized so that it is possible to form the film that is elaborate on an atomic level as compared with the deposited film.
However, a drawback has been that the thin film formed by the conventional sputtering method as described above is subjected to polycrystallization, i.e. a number of crystal plane orientations appear on the film surface and spaces are formed among crystals, and therefore, a completely elaborate film cannot be formed so that the reflectance has an upper limit of about 92% like the deposited film.
The present inventors recognize that it is necessary to match the plane orientations in order to further improve the reflectance.
It is known that aluminum easily reacts with oxygen in the air to form a stable natural aluminum oxide coating film having a thickness of about 10 nm and becomes reluctant to corrode by the formation of the natural aluminum oxide coating film.
However, since the natural aluminum oxide coating film is formed by the molecular oxidation reaction, the film is not uniformly oxidized to the inside so that the film thickness becomes unequal and, further, since the chemical composition near the aluminum interface after the oxidation is not stoichiometric, i.e. lacking oxygen atoms, it is the film with many defects and is not fully capable of stress relaxation and, as a result, the inside aluminum layer is subjected to corrosion and therefore it cannot sufficiently serve as a protective layer.
Further, in order to stabilize an aluminum thin film, a method is to form a thick passive film by anodic oxidation or the like. Aluminum formed with an oxide film by the anodic oxidation method is called alumite and widely used.
However, since the aluminum anodically oxidized film is formed by the oxidation caused by oxygen ions and is a porous oxide film having microholes due to a manufacturing method, a stress remains to cause occurrence of microcracks due to a change in external environment such as temperature and humidity, and moisture or the like infiltrates into the microholes. Therefore, the aluminum anodically oxidized film cannot completely prevent corrosion of an aluminum layer.
Also a method exists which improves corrosion resistance by plasma-polymerizing an organic compound such as trimethyldisiloxane and using it as a coating. However, this method not only reduces the reflectance and increases the processes and members in number to raise the cost, but also makes it impossible to accurately reproduce the fine surface shape for controlling the light reflection direction, and therefore, is not appropriate.
To this end, it has been necessary to directly form a uniform and elaborate passive coating film on an aluminum thin film.
It is an object of this invention to provide a visible light reflecting plate or film having an aluminum thin film with a reflectance higher than that of a conventional aluminum thin film.
It is another object of this invention to provide a visible light reflecting plate or film using an aluminum thin film that facilitates a direction control of reflecting light.
It is still another object of this invention to provide a visible light reflecting plate or film with an aluminum thin film having a uniform corrosion-resistant surface passive film.
It is yet another object of this invention to provide a visible light reflecting plate or film with an aluminum thin film suitable for a large-size reflecting plate such as a reflecting plate for a large-size flat panel liquid crystal display backlight unit.